Optical fibers have found increasing uses in industrial, scientific, and military applications. Conventional optical fibers guide light passing through them using the principles of total internal reflection. Total internal reflection (TIR) occurs when light travels through a material having a high index of refraction n and strikes an interface between that material and a material having a lower value of n. If the angle of incidence of the light on the interface is greater than some angle, known as the “critical angle,” θc, the light cannot pass through the interface into the lower-refractive material but instead is reflected back into the higher-refractive material. Thus, for optical glass fibers, the principle of total internal reflection requires that the inner core of the fiber have a higher index of refraction than the outer cladding. However, due to the nature of the materials used, such conventional fibers still exhibit some absorption and scattering of the light traveling through them and can therefore suffer some loss as the signal travels through the fiber.
More recently, microstructured optical fibers have been developed in an attempt to improve the transmission and reduce the leakage of light traveling therethrough. These microstructured optical fibers include hollow core photonic band gap (HC-PBG) fibers. Like conventional optical fibers, HC-PBG fibers have a three-layer structure comprising a core area, an intermediate cladding surrounding the core area, and a jacket made of solid glass surrounding the cladding. However, in HC-PBG fibers the cladding is not solid as in conventional optical fibers, but instead comprises a microstructured region having a periodic arrangement of glass and holes, which confines the light to the core of the fiber.
HC-PBG fibers operate on the principle of two-dimensional photonic bandgap confinement, a condition which prohibits the propagation of specific wavelengths within the photonic bandgap cladding region. The existence of a photonic bandgap is governed by the wavelength of interest, and the transverse dielectric function of the fiber. The transverse dielectric function describes the refractive index of a cross-section of the fiber and is governed by the refractive index of the glass, the shape and location of the holes, the hole diameter and pitch, the ratio of which governs the air fill fraction, and the lattice arrangement, i.e., triangular, square, etc. Since the light in HC-PBG fibers is confined primarily to the air void in the hollow core, and not the glass as in conventional TIR fibers, both signal loss and light-induced fiber damage due to transmission through a solid glass core, are reduced. This enables HC-PBG fibers to transmit higher energy signals over longer distances.
Microstructured optical fibers have been fabricated from silica and other glasses, and their design and manufacture have been described in the literature. For example, see R. F. Cregan et al., “Single-mode photonic band gap guidance of light in air,” Science, Vol. 285, pp. 1537-1539 (1999) (describing photonic band gap (PBG) guidance of light through optical fiber comprising tubes of silica glass arranged in a periodic pattern); S. Barkou et al., “Silica-air photonic crystal fiber design that permits waveguiding by a true photonic bandgap effect,” Optics Letters, Vol. 24, No. 1, pp. 46-48 (1999) (describing silica glass fiber having air holes arranged in a honeycomb pattern with an additional central air hole in which light having specific wavelengths can be confined); and N. Venkataraman, et al., “Low loss (13 dB/km) air core photonic band-gap fibre,” ECOC, Postdeadline Paper PD1.1, September, 2002 (describing low signal loss properties of silica glass HC-PBG fibers).
The periodic layered structure of holes and glass in the HC-PBG fiber creates a photonic band gap that prevents light from propagating in the structured region, i.e. a two dimensional band gap confinement as noted above. As such, light is confined to the hollow core. The core of the fiber takes the place of a defined number of holes in the periodic structure. For example, the core may take the place of seven small holes in the periodic structure, thus this arrangement is referred to as a 7-cell HC-PBG fiber. Similarly, a 19-cell HC-PBG fiber has a larger core, which takes the place of 19 small holes in the periodic structure. Typically, the periodicity of the holes is on the scale of the wavelength of light being transmitted and the outer glass is used for providing mechanical integrity to the fiber. Because light traveling in the hollow core experiences greatly reduced losses, longer path lengths may be fabricated. Also, non-linear effects experienced, for example in solid core devices, are negligible in HC-PBG fibers and damage thresholds will be higher, such that higher power laser energy can be transmitted through the fiber, making it suitable for military as well as commercial applications. Further, due to the fact that light is guided in the hollow core, an analyte disposed therein will have maximum interaction with light, unlike traditional evanescent sensors.
Microstructured optical fibers, though conventionally fabricated using glass, also can be made from non-silica glass such as chalcogenide glasses. See, e.g., U.S. Patent Application Publication No. 2005/0074215; U.S. Patent Application Publication No. 2006/0230792; U.S. Patent Application Publication No. 2010/0303429; and U.S. Pat. No. 7,295,740, each of which shares at least one inventor in common with the present invention.
The periodicity of the holes, the air fill fraction, and the refractive index of the glass dictate the position of the photonic band gap, i.e. the transmission wavelengths guided through the hollow core. Such microstructured optical fibers are typically made using a preform comprising an outer shell and a number of hollow tubes arranged in a periodic structure, with a hollow core, which is then drawn into the final fiber. See U.S. Pat. No. 6,847,771 (describing microstructured optical fibers and fabrication of such fibers from optical fiber preforms).
The preform is typically comprised of a central core structured region, typically made by stacking microtubes or microcanes, which is then inserted into a supportive outer jacket or tube. In the preform, a number of glass microtubes placed in a periodic arrangement between the core and the outer jacket form the cladding. Such microtubes are hollow tubes having an opening, i.e., a hole, extending through their entire length, while microcanes may be solid or hollow. The arrangement of the microtubes and/or microcanes creates a periodic structure of glass and holes in the cladding which affects the transmission of light therethrough. The preform is then drawn to create the optical fiber.
However, because the microtubes and/or microcanes comprising the cladding do not always fit together perfectly, the assembly process inevitably introduces gaps, or voids, at the interfaces between the microtubes/microcanes or between the cladding area and the outer jacket. Additionally, conventional processes rely on stacking the microtubes or microcanes, by hand or otherwise, which may result in errors in the periodicity of the fiber produced, as well as additional gaps or voids. Such “interfacial voids” extend longitudinally through the entire length of the preform and are connected to the ambient atmosphere outside the preform via the preform ends. Many of these voids can be eliminated during the fiber drawing or other heat treatment step as the tubes are drawn closer together, but often some of these voids remain as “interstitial voids.” These interstitial voids are not connected to the atmosphere outside the fiber but are trapped within the fiber.
The presence of both the interfacial and interstitial voids is undesirable. The interfacial voids run the entire length of the preform and have a size similar to that of the intended holes in the structured region and so can make fiberization difficult. This is especially true for specialty oxide and non-oxide glasses where the vapor pressure during fiberization may be sufficient to prevent collapse of these interstitial voids. Furthermore, the accuracy of the periodicity and position of the intended holes is critical to attaining band gap guidance in the fiber, and is adversely affected by the presence of such “stray” holes in the fiber caused by incorrect tube positioning and tube slippage during fiberization, which can destroy the ability of the fiber to perform properly. As such, interstitial voids are common deficiencies of the tube stacking preform method.
Conventional processes attempt to reduce or eliminate the number of such interstitial voids by using a two-step process, in which the tubes in the preform are consolidated prior to fiber drawing. However, this two-step process still leaves an undesirable number of interstitial voids in the finished fiber.
Another process involves the application of a vacuum during drawing to reduce the presence of voids. However, this may cause soot to accumulate on the preform that can interfere with optical performance of the HC-PBG fiber. Other processes have been attempted, but no process has achieved the desired result of rendering a HC-PBG fiber devoid of interstitial spaces that degrade fiber performance. What is desired is a method to produce a HC-PBG fiber preform that does not suffer from the presence of voids or gaps that degrade the performance of the resulting optical fiber.